1. Field of the Invention
The present invention relates to a rotation detecting device that employs a plurality of magnetoresistive elements and a biasing magnet to be used for an engine control system, antilock brake system (ABS) of a vehicle, etc.
2. Description of the Related Art
Engine ignition timing is usually determined on the basis of a crankshaft angle and a camshaft angle. For example, the camshaft of a four-cycle multi-cylinder engine makes one turn when the crankshaft makes two turns. Therefore, cylinder discrimination data are inputted made while the cam makes one turn, and the ignition timing data are inputted while the crankshaft makes one turn.
U.S. Pat. No. 6,452,381B1 or its counterpart JP-11-237256A2 discloses a rotation detecting device, which is shown in FIG. 15 of this application. This prior art rotation detecting device includes a pair of MRE bridge circuits 66 and 69 of magnetoresistive elements (MRE), which are symmetrically disposed with respect to a center line that extends through the magnetic center of a gear type rotor 62 so that detecting axis of one of the bridge circuits is 90 degrees different from the detecting axis of the other. The MRE bridge circuit 66 is comprised of a pair of series-connected MRE 64 and MRE 65, and the MRE bridge circuits 69 is comprised of a pair of series-connected MRE 67 and MRE 68. The gear type rotor 62 has three teeth 62a and valley portions 62b formed between the teeth 62a. The MRE bridge circuits 66, 69 are respectively disposed at positions shifted from the magnetic center of the biasing magnet 63 in the rotating direction. When the gear type rotor 62 rotates about its center axis, the MRE bridge circuit 66 provides an output signal at the joint of the MRE 64 and the MRE 65, and the MRE bridge circuit 69 provides an output signal at the joint of the MRE 67 and the MRE 68. Both output signals are inputted into a differential amplifier 70, which amplifies the difference between the inputted signals to provide an output signal that corresponds to a rotation angle of the rotor.
In the above rotation detecting device, a minimum air-gap characteristic point may change if one of the teeth of the gear type rotor 62 is different in shape from the others. Incidentally, the minimum air-gap characteristic point is a point where the differential amplifier 70 provides the same output signal when the biasing magnet rotates the same rotation angle although the air gap between the gear type rotor 62 and the MRE bridge circuits 66, 69 changes.
U.S. Pat. No. 2003 0173955 or its counterpart JP-A 2003-269995 proposes to keep the air-gap minimum characteristic point constant by repeating differential amplification. However, it is very difficult to fix the magnetic sensors to an accurate position of an integrated circuit and to uniformly keep the air gap between the biasing magnet and the magnetic sensor.
When the magnetic sensor 63 is fixed at a normal position as shown in FIG. 16, the differential amplifier 70 provides a normal output signal as shown in FIG. 16C. However, the differential amplifier 70 provides an abnormal signal as shown in FIG. 16D if the magnetic sensor 63 is fixed at a position shifted vertically (in Y-direction), horizontally (in X-direction) or angularly (by θ) from the normal position as shown in FIGS. 16B, 16E and 16F. This abnormal signal may cause improper ignition timing.